In U.S. Pat. No. 3,821,935 granted July 2, 1974 a hatch cover is provided with a handwheel having a threaded shaft extending through the hatch cover. A hub is threaded onto the shaft having a plurality of radially extending locking arms which engage fixed lugs on the hatch coaming. A guide plate depending from the cover includes a cam slot which receives a follower extending from the hub. Rotation of the handwheel causes rotation and vertical movement of the locking arms into engagement with the lugs on the hatch coaming. The later part of this movement causes a seal on the cover to be urged into engagement with a lip on the coaming. To open the cover the handwheel is rotated in the opposite direction until the locking arms clear the locking lugs. The cover is then pivoted about a hinge to the open position.
This construction is disadvantageous because of the difficulty of obtaining a satisfactory seal between the threaded shaft and the cover, the large number of turns of the handwheel, at least about 13, to move the locking arms between open and closed position, and because of cost. Furthermore the internal friction between the threaded shaft and hub may result in some contamination of lubricant and/or ground metal.
Also the force urging the seal into engagement with the coaming is dependent upon the number of turns applied by the operator. If an insufficient number of turns is applied by the operator, the cover may not remain properly seated in transit.